It has been found that polymeric materials can be drawn into thin, transparent films. When this is done, however, it is difficult to tell whether the film has been properly made, or whether it has flaws, especially if the film has multiple layers. Many critical flaws are not visible. Hand calibration of thickness is not feasible. The standard analytical instrument for measuring oxygen permeability, as described in U.S. Pat. No. 5,107,696, can only detect average permeability over an area typically five square inches.
Various methods of measuring the presence of oxygen are known for use in various systems. Liquid systems are discussed in U.S. Pat. No. 4,659,674 issued to Bauman et al., Apr. 21, 1987, which discloses an ion-specific electrode. The possibility of determining oxygen permeation via pH change is discussed but only overall permeability is disclosed, and surface flaws such as pinholes in a barrier could not be detected.
The amount of oxygen in a gaseous stream has also been measured. For example, U.S. Pat. No. 3,725,658, issued to Stanley et al., Apr. 3, 1973, relates to a medical oxygen analyzer. It discloses an apparatus and method for continuously detecting rapid changes in the oxygen content of a gas stream; that is, a total response time of not more than 0.1 seconds per measurement. The reference relies on the use of a fluorescent material such as pyrene, coronene and p-terphenyl whose fluorescence is partially quenched by the presence of oxygen. Elaborate mechanical support is required. There is no spatial resolution of oxygen flow.
Oxygen detectors have been used in packaging. U.S. Pat. No. 4,526,752, issued to Perlman, Jul. 2, 1985, relates to a tamper-resistant package. A dye, such as methylene blue, which is colorless in the reduced state and becomes colored upon exposure to oxygen is dissolved in water along with a volatile reducing agent. The reducing agent is removed, along with the water, preferably under vacuum, and the package is sealed. If the package is broken, the dye will become colored upon exposure to air. The change in color of the package is irreversible.
Another type of oxygen detector is used in U.S. Pat. No. 3,768,976, issued to Hu et al. Oct. 30, 1973, which relates to a temperature-time indicator for food packaging. The indicator is a film package that contains an aqueous solution of a redox dye such as sodium anthraquinone beta-sulfonate. The dye in its reduced state is dark red and obscures a warning message. As oxygen permeates into the package in an amount which is dependent on temperature and time, the dye fades and the warning message is revealed. This system is not reversible, and spatial resolution of the rate of oxygen permeation is not disclosed or discussed.
Similarly, U.S. Pat. No. 4,169,811, issued to Yoshikawa Oct. 2, 1979 discloses an oxygen indicator which is a dye, a base, and a reducing agent. The dye has one color in an anaerobic environment and another color in an aerobic environment. These dyes are derivatives of methylene blue. It is disclosed that these dyes require the presence of water or an alcohol in order to function. The reducing agents are disclosed to be saccharides, dithionites and ferrous compounds. The oxygen sensitivity is disclosed to be as low as 0.1% [column 6, line 65].
A probe is disclosed in U. K. Patent Application 2132348A, which relates to the use of platinum group metal complexes which luminesce when excited by visible or UV light, and which are quenched by oxygen and other materials. A sensor, which incorporates the metal complex in a carrier, which must be permeable to oxygen and relatively impermeable to other quenchers is exposed to the environment and oxygen permeates the carrier and partially quenches the fluorescence of the metal complex. The quenching-related decrease in intensity or lifetime of luminescence is measured and correlated to the presence of oxygen. The precision and accuracy is about 2 percent. Spatial resolution of oxygen permeability is not disclosed. The use of a sensor akin to pH paper is discussed, which is said to yield only semi-quantitative or qualitative oxygen monitoring (Col. 8, lines 116-126).
The difficulty with many indicators is that they are not physically compatible with the most likely carriers. U.S. Pat. No. 4,657,736, issued to Marsoner et al. Apr. 14, 1987, addresses this point, disclosing that fluorescent indicators can be reacted with tertiary butyl chloride to render them compatible with silicone polymer carriers to avoid having the indicator crystallize out of the polymer.
What is needed is a method of detecting oxygen transmission through a barrier that is useful for quality control in day-to-day manufacture of polymer sheets and other objects, and for design and development of new oxygen barrier materials. The method should be relatively quick and be both qualitative and quantitative. It should also be activated on demand and capable of detecting manufacturing defects such as streaks and pinholes.
Although this application is written in terms of a specific end use, one of ordinary skill in the art will readily recognize that it is a general tool for detecting cracks and pinholes wherever oxygen might be used as an indicator. For example, it could be used to detect flaws in sheets of aluminum foil. In that case, oxygen permeability per se might not be the primary interest, if one is interested in the physical integrity of the foil. Similarly, the integrity of opaque or tortuous path type materials such as ceramics could be tested as well.
The inventors have found that a system based on the reaction of a redox indicator can be used to measure oxygen transmission in great physical detail, that is, make an image of a barrier's permeability. This Low Oxygen Transmission Imaging System ("LOTIS") can be used in both qualitative and quantitative modes.
It is, therefore, an object of this invention to provide a method and apparatus for detecting the rate of permeation of oxygen through a barrier at various locations.
Another object is to provide as the detecting component for oxygen analysis a stable sheet comprising a redox compound dissolved in a solvent or carrier, which sheet is supported on a substrate and is re-usable.
A particular advantage of the present invention is that the redox compound and carrier can be stored indefinitely under ambient, that is, oxygenated conditions. Previous systems using chemical reductants and redox dyes had to be prepared and used fresh due to their oxygen reactivity.